1. Field of the Invention
This invention relates to a device for controlling the ignition and fuel injection of an internal combustion engine, of the type having a digital computer system programmed to compute the ignition advance and the injection time as a function of the speed and the load of the engine.
2. Description of the Prior Art
In any device of the above-noted type, it is necessary to measure the engine speed and to know with good precision the instantaneous angular position of the crankshaft so as to trigger the ignition at the computed angle as a function of the measured speed and load.
For this purpose, it is known how to use a target fixed in rotation with the crankshaft and exhibiting at its periphery one or more irregularities passing before a sensor that supplies a synchronization signal Sy at each passage of a piston by a predetermined position such as a top dead center (TDC) and/or by a bottom dead center (BDC). This synchronization signal makes it possible, between two consecutive TDCs or BDCs, to measure the speed with a precision suitable for computation of the angle of advance and the injection time and to trigger the start of the injection at the desired moment, generally at the top dead center, but it is not suitable for triggering the ignition which, for a given stroke, is offset by several dozen degrees in relation to the preceding TDC or BDC.
One solution which consists in performing an electronic interpolation of the synchronization signal is not satisfactory because, in case of sharp acceleration or deceleration of the engine, the electronically restored angular position does not coincide with the true angular position of the crankshaft.
Another solution consists in using a second target also fired in rotation with the first and provided at its periphery with several dozen regularly spaced teeth and hollows which pass before a second sensor. Further, since the angular resolution necessary for triggering the ignition would lead to a number of teeth and hollows incompatible with a large series production, an electronic interpolation of the signal coming from the second sensor is preferably performed. The synchronization of the interpolation signal is done at each detection of a new tooth and/or a new hollow, which makes it possible to avoid any significant error in case of sharp acceleration or deceleration of the engine. However, this second solution has the drawback of being costly on the industrial level because it requires the use of two sensors and two targets which, in addition, must be perfectly angularly adjusted in relation to one another.
U.S. Pat. No. 4,321,580 describes a process which makes it possible to solve this problem by measuring the speed and marking the angular position of the crankshaft by means of a single sensor in front of which passes a target equipped at its periphery with a series of teeth and hollows and from which at least one tooth has been eliminated to constitute an absolute reference. U.S. Pat. No. 4,367,710 describes a specific ignition computer that uses this process and makes its possible to obtain the synchronization signal Sy, an image tooth signal Sd of the teeth of the target and an interpolation signal or frequency speed signal V n times greater than Sd. From these signals, the computer computes the angle of ignition advance, the conduction time of the coil and consequently controls the coil. This computer can consist of the integrated circuit SN 96 528 of the Texas Instruments Company.
Since it involves a specific circuit, this computer is not programmable and therefore is not suitable to assure the additional function of controlling the fuel injection.
On the other hand, the processing of the signal, if it is desired to have sufficient dynamics to measure the rotation speed and an accurate marking of the TDC and/or BDC, the computation of the conduction angles of the coil and the angles of ignition advance, and the computation of the injection time would make it necessary to use a priori a fast and powerful and consequently costly, microprocessor.
Another solution described in the article titled "Engine Controls Become More Cost Effective" in the magazine Automotive Engineering, volume 89, No. 8, August 1981, consists in using two single chip microprocessors working in parallel, one for the ignition, the other for the injection, from a certain number of parameters some of which are common. A first drawback of this solution resides in the fact that the marking of the instantaneous angular position of the crankshaft is assured by an electronic interpolation of the synchronization signal that produces a pulse theoretically every 0.35.degree. of rotation. Unfortunately, as previously shown, this restored angular marking is proved erroneous when the crankshaft is subjected to sharp accelerations or decelerations, as is the case at the low speeds of slowing down or at starting. Moreover, this solution with two microprocessors nevertheless remains costly because for a very large series production the cost of even a single chip microprocessor is greater than a microprocessor with a specific integrated circuit.